A. Field of the Invention
Broadly speaking, this invention relates to protection switching for microwave radio. More particularly, in a preferred embodiment, this invention relates to a protection switching system employing frequency-agile-repeaters.
B. Discussion of the Prior Art
As is well known, microwave radio systems are widely employed in the telecommunications industry. For example, the microwave radio system known as TD-3 operates in the 4 GHz common carrier band and derives up to 12, two-way radio channels, each capable of carrying up to 1500 voice-grade telephone circuits.
Obviously, the failure of an operational microwave channel, even for a few seconds, would have serious consequences and for that reason it has become standard in the industry to provide a protection switching arrangement for every operational microwave system. The most common form of protection switching is the so-called, "hot standby" arrangement wherein one or more of the working microwave channels are withheld from service and kept idle, ready to be brought into service the moment that one of the working channels fails or becomes marginal.
The increasing congestion in the common carrier bands, and recent rulings by the Federal Communications Commission, have led to a re-examination of this approach to the provision of protection switching. One proposal is to use all the channels in a given system for traffic purposes and to provide a duplicate set of standby equipment for each working channel, switching to the standby equipment if and when the regular equipment fails. While the above approach is efficient in terms of spectrum usage, it is extremely inefficient in terms of cost and space requirements and is, thus, not economically viable. Another proposal is to provide only one spare repeater and to substitute that repeater for a failed repeater, as required. Of course, each time that the standby repeater was placed in service it would be necessary to re-tune the repeater to the transmitting and receiving frequencies of the failed repeater. Such re-tunable repeaters, known in the industry as frequency-agile-repeaters, are commercially available and can be easily re-tuned, by remote control if desired, well within the time limits established for the protection switching scheme.
Unfortunately, there is more to a multichannel microwave system than the repeaters. Typically, such systems also include channel-separating networks, channel-combining networks, filters, circulators, etc., all of which must also be carefully tuned to the frequency of the microwave channel that they are associated with. This tuning is a slow and painstaking operation and cannot be done fast enough to meet the switching limits speed of the protection switching scheme. In other words, while a frequency-agile-repeater per se can be re-tuned fast enough to meet the requirements of a protection switching system, the associated "plumbing," in general, cannot.